feat(fase2): add Flow schemas and Flow Engine core

API Gateway:
- Add Pydantic schemas for Flow API (create, update, response, nodes, edges)

Flow Engine:
- Add FlowContext for variable management and interpolation
- Add FlowEngine for executing chatbot flows
- Support node types: trigger, message, buttons, wait_input, condition, set_variable

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/api-gateway/app/schemas/flow.py

@@ -0,0 +1,74 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from uuid import UUID
+from datetime import datetime
+from app.models.flow import TriggerType
+
+
+class NodeData(BaseModel):
+    label: str
+    type: str
+    config: dict = {}
+
+
+class FlowNode(BaseModel):
+    id: str
+    type: str
+    position: dict
+    data: NodeData
+
+
+class FlowEdge(BaseModel):
+    id: str
+    source: str
+    target: str
+    sourceHandle: Optional[str] = None
+    targetHandle: Optional[str] = None
+
+
+class FlowCreate(BaseModel):
+    name: str
+    description: Optional[str] = None
+    trigger_type: TriggerType
+    trigger_value: Optional[str] = None
+
+
+class FlowUpdate(BaseModel):
+    name: Optional[str] = None
+    description: Optional[str] = None
+    trigger_type: Optional[TriggerType] = None
+    trigger_value: Optional[str] = None
+    nodes: Optional[List[dict]] = None
+    edges: Optional[List[dict]] = None
+    variables: Optional[dict] = None
+    is_active: Optional[bool] = None
+
+
+class FlowResponse(BaseModel):
+    id: UUID
+    name: str
+    description: Optional[str]
+    trigger_type: TriggerType
+    trigger_value: Optional[str]
+    nodes: List[dict]
+    edges: List[dict]
+    variables: dict
+    is_active: bool
+    version: int
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class FlowListResponse(BaseModel):
+    id: UUID
+    name: str
+    trigger_type: TriggerType
+    is_active: bool
+    version: int
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True